Dilation system and method of using the same

ABSTRACT

A method of forming an access opening through a psoas muscle to a patient&#39;s spine includes laterally inserting a stimulating dilator into the psoas muscle. The stimulating dilator has a stimulation channel formed in an outer surface thereof. An electrical pulse is transmitted into the stimulating dilator to locate a position of a nerve in the patient&#39;s psoas muscle. The stimulating dilator is laterally inserted through the psoas muscle and toward the patient&#39;s spine in a way that avoids the nerve. A stimulating probe is inserted into the stimulation channel along the outer surface of the stimulating dilator while transmitting an electrical pulse into the stimulating probe to verify the position of the nerve.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/964,561, filed on Apr. 27, 2018; which is a continuation of U.S.patent application Ser. No. 15/680,433, filed on Aug. 18, 2017; which isa continuation of U.S. patent application Ser. No. 15/202,738, filed onJul. 6, 2016, now U.S. Pat. No. 9,737,290; which is a continuation ofU.S. patent application Ser. No. 12/681,671, filed on Apr. 5, 2010, nowU.S. Pat. No. 9,387,009, issued on Jul. 12, 2016; which is a U.S.National Stage Entry of PCT/US08/78927, filed on Oct. 6, 2008; whichclaims the benefit of U.S. Provisional Application No. 60/977,882, filedon Oct. 5, 2007, entitled “ADJACENT OR LATERAL DILATOR AND METHOD OFUSING THE SAME;” the entire contents of each being hereby expresslyincorporated by reference herein.

BACKGROUND OF THE INVENTION

Generally speaking, sequential dilation systems enable a surgeon to makean initial incision and gradually increase the size of the incision bysequential insertion of increasingly larger dilators. Sequentialdilation is preferably able to reduce tissue damage and associatedtrauma to speed patient recovery time. In addition, dilation is utilizedto prepare a surgical path to a surgical site and a stimulator may beutilized with the dilator to direct the dilator along a path that avoidsspecific areas of the patient's anatomy, such as neural elements ornerves.

SUMMARY OF THE INVENTION

A preferred embodiment of the present invention relates generally tominimally invasive surgical procedures and apparatus and, moreparticularly, to a dilation system and related methods for directionaldilation of an incision for use in creating an access opening to apatient's spine. More specifically, the present invention relates to adilation system and related methods that are able to laterally access alumbar region of a patient's spine through the patient's psoas muscle.In accordance with one aspect of the present invention, the neuralelements or nerves of the psoas muscle are preferably mapped using astimulating probe, thereby defining a safe zone of passage. Thestimulating probe is inserted through the psoas muscle and toward orinto the intervertebral disc space. Directional dilators may be used todilate the psoas muscle to substantially separate tissue on only oneside of the stimulating probe. That is, directional, sequential dilatorsmay be inserted to dilate the psoas muscle, for example, on the anteriorside of the stimulating probe while substantially leaving the psoasmuscle intact on the posterior side of the stimulating probe.Specifically, the directional, sequential dilators may be utilized todirectionally dilate tissue away from a neural element or nerve in thepatient's body that is identified by the stimulating probe such that theneural element or nerve is not disturbed or damaged by the dilationprocess or other surgical procedures that may occur following dilation.

Alternatively, the dilation system and method may include a bluntstimulating dilator including at least one channel formed in an outersurface. The channel receives a stimulating probe that is used to mapthe neural elements or nerves of the psoas muscle and define a safe zoneof passage to the patient's spine. The stimulating dilator is insertedthrough the psoas muscle and toward or into the intervertebral discspace. A stimulating probe is then inserted into the channel formed inthe outer surface of the stimulating dilator in order to verify theneural elements or nerves.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the application, will be better understoodwhen read in conjunction with the appended drawings. For the purposes ofillustrating the dilation system and methods of the present application,there is shown in the drawings preferred embodiments. It should beunderstood, however, that the application is not limited to the precisearrangements and instrumentalities shown. In the drawings:

FIG. 1 illustrates a side elevational view of a dilation system inaccordance with a first preferred embodiment the present invention,which will generally be referred to herein as a directional sequentialdilation system;

FIG. 2 illustrates a magnified perspective view of the distal end of thedirectional sequential dilation system shown in FIG. 1;

FIG. 2A illustrates an exploded view of the distal end of thedirectional sequential dilation system shown in FIG. 2;

FIG. 3 illustrates a cross-sectional view of the directional sequentialdilation system of FIG. 1, taken along line 3-3 of FIG. 1;

FIG. 4 illustrates a front elevational view of the directionalsequential dilation system shown in FIG. 1 including a schematicrepresentation of a retractor that may be used in connection with thedirectional sequential dilation system;

FIG. 5 A illustrates a side view of a first directional dilator used inconnection with the directional sequential dilation system shown in FIG.1;

FIG. 5B illustrates a front view of the first directional dilator shownin FIG. 5 A;

FIG. 6A illustrates a side view of a second directional dilator used inconnection with the directional sequential dilation system shown in FIG.1;

FIG. 6B illustrates a front view of the second directional dilator shownin FIG. 6A;

FIG. 7 illustrates a side elevational view of a dilation system inaccordance with a second preferred embodiment of the present invention,which will generally be referred to herein as a blunt stimulatingdilation system;

FIG. 8 illustrates a top plan view of the blunt stimulating dilationsystem of FIG. 7;

FIG. 9 illustrates a magnified, top perspective view of a proximal endof the blunt stimulating dilation system of FIG. 7;

FIG. 10 illustrates a magnified bottom perspective view of a distal endof the blunt stimulating dilation system of FIG. 7;

FIG. 11 illustrates a top plan view of a dilation system in accordancewith a third preferred embodiment of the present invention, which isalso comprised of a blunt stimulating dilation system; and

FIG. 12 illustrates a magnified, top perspective view of a proximal endof the blunt stimulating dilation system of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenienceonly and is not limiting. The words “right”, “left”, “top” and “bottom”designate directions in the drawings to which reference is made. Thewords “inwardly” and “outwardly” refer to directions toward and awayfrom, respectively, the geometric center of the directional sequentialand blunt stimulating dilation systems and designated parts thereof. Thewords, “anterior”, “posterior”, “superior”, “inferior” and related wordsand/or phrases designate preferred positions and orientations in thehuman body to which reference is made and are not meant to be limiting.The terminology includes the above-listed words, derivatives thereof andwords of similar import.

Certain exemplary embodiments of the invention will now be describedwith reference to the drawings. In general, such embodiments relate todilation systems for accessing a patient's spinal column and,preferably, for laterally accessing the lumbar region of the patient'sspine.

As generally understood by one of ordinary skill in the art, thedilation systems will be described in connection with accessing thespine to perform a surgical procedure, but the dilation systems willfind use not only in orthopedic surgery, but in other surgicalprocedures in which a surgeon wishes to gain access to an internalcavity by cutting the skin and going through the body wall in order tokeep the incision spread apart so that surgical instruments can beinserted. For example, the dilation systems may be used for anteriorlyor posteriorly accessing the spine, for accessing the thoracic orcervical region of the spine, or for accessing nearly any other part ofthe body.

Referring to FIG. 4, generally speaking, during a lateral approach to apatient's spine 2, a psoas muscle 4, which is located on either side ofthe spine 2, is preferably separated in order to access the spine 2 and,in particular, an intervertebral disc space 6 or one or more vertebralbodies 8 within a patient's spinal column. It is desirable to avoidneural elements or nerves 9 of the lumbar plexus that lie within thepsoas muscle 4 during such procedures. The anterior third of the psoasmuscle 4 is typically considered a safe zone for muscle separation.

The neural elements or nerves 9 of the psoas muscle 4 are preferablymapped using a stimulating probe 20. In this manner, the most posteriorneural or nerve free area of the psoas muscle 4 is preferably locatedand identified. The stimulating probe 20 may then be inserted throughthe psoas muscle 4 via the most posterior neural or nerve free tissuearea or through nearly any other region that is free of neural elementsor nerves 9 and toward the spine 2 or into the intervertebral disc space6 in order to initiate safe tissue separation of the psoas muscle 4.Directional dilators 30, 40 in accordance with the first preferredembodiment of the present invention may be used to dilate the muscleseparation. In particular, the directional dilators 30, 40 may be usedto primarily separate tissue on one side of the stimulating probe 20(shown as cranial side), preferably on the anterior side of thestimulating probe 20 (e.g., the safe zone as initially identified andmarked by the stimulating probe 20). That is, by using the directionalsequential dilators 30, 40, the tissue on one side of the stimulatingprobe 20 may be moved while substantially limiting movement of thetissue on the opposite side of the stimulating probe 20. In comparison,concentric dilators separate the muscle radially and, as such, dilatetissue on both sides of the stimulating probe. This in turn may impingeon neural elements or nerves 9 located outside of the safe zone.

Referring to FIGS. 1-6B, a first preferred embodiment of a dilationsystem of the present invention is comprised of a directional sequentialdilation system 10. The directional sequential dilation system 10preferably includes a stimulating probe 20, a first directional dilator30 and a second directional dilator 40. The directional sequentialdilation system 10 may include more or less dilators such as, forexample, one, three, four, etc. The stimulating probe 20 can be anyprobe now or hereafter known for transmitting an electrical pulse. Thestimulating probe 20 preferably includes a probe tip 20 a and alongitudinal probe axis 21. The first directional dilator 30 preferablyincludes a first longitudinal axis 31, an outer surface 32, a proximalend 33, a distal end 34 and a first bore 35 extending from the proximalend 33 to the distal end 34. The first directional dilator 30 alsopreferably includes a first tip 30 a at the distal end 34 through whichthe first longitudinal axis 31 extends. The first bore 35 has a firstbore axis 36 that extends from a proximal end to a distal end of thefirst bore 35. The first longitudinal axis 36 is preferably offset orlocated a first offset distance A from the first longitudinal axis 31.The first directional dilator 30 also preferably includes a firstchannel 38 formed in the outer surface 32 thereof. The first channel 38is preferably in communication with the first bore 35 along the entirelength of the first bore 35. In use, the first bore 35 and the firstchannel 38 removably receive the stimulating probe 20 in an assembledconfiguration (e.g., when the stimulating probe 20 is slidably receivedwithin the first bore 35 of the first directional dilator 30) so that asurgeon can stimulate the first directional dilator 30. The probe axis21 of the stimulating probe 20 is preferably coaxial with the first boreaxis 36 of the first directional dilator 30 in the assembledconfiguration.

Similarly, the second directional dilator 40 preferably includes asecond longitudinal axis 41, an outer surface 42, a proximal end 43, adistal end 44 and a second bore 45 extending from the proximal end 43 tothe distal end 44. The second bore 45 preferably has a second bore axis46 that extends from the proximal end 43 to the distal end 44. Thesecond directional dilator 40 also preferably includes a second tip 40 aat the distal end 44 through which the second longitudinal axis 41extends. The second bore axis 46 of the second bore 45 is offset orlocated a second offset distance B from the second longitudinal axis 41.The outer surfaces 32, 42 of the first and second directional dilators30, 40 are preferably coated to prevent electrical leakage during use,as will be apparent to one having ordinary skill in the art. The seconddirectional dilator 40 also preferably includes a second channel 48formed in the outer surface 42 thereof that is in communication with thesecond bore 45. In use, the second bore 45 and the second channel 48receive the first directional dilator 30 therein in the assembledconfiguration (e.g., when the first directional dilator 30 is slidablyreceived within the second bore 45 of the second directional dilator40). The first longitudinal axis 31 of the first directional dilator 30is preferably coaxial with the second bore axis 46 of the seconddirectional dilator 40 when in the assembled configuration.

Because the first and second bore axes 36, 46 of the first and secondbores 35, 45 are offset from the first and second longitudinal axes 31,41 of the first and second directional dilators 30, 40, respectively,inserting the first directional dilator 30 over the stimulating probe 20and then the second directional dilator 40 over the first directionaldilator 30 causes each sequential dilator to “directionally” dilate theopening formed in the patient preferably away from any neural elements,nerves 9 or other anatomic structure on the opposite side of thestimulating probe 20, as will be described in greater detail below.

Moreover, incorporation of the first and second channels 38, 48 enablesthe first and second directional dilators 30, 40 to be more closelynested together and thus, substantially eliminate the “cookie cutter”effect that is realized when using multiple concentric dilators ofincreasing inner bore size.

The first directional dilator 30 preferably includes a plurality offirst depth indicators 37 located on the outer surface 32 thereof (asbest shown in FIGS. 5 A and 5B). The plurality of first depth indicators37 extend, on the outer surface 32 of the first directional dilator 30,generally perpendicular to the first longitudinal axis 31. The pluralityof first depth indicators 37 indicate to the surgeon the variousdistances between the first tip 30 a formed at the distal end 34 of thefirst directional dilator 30 to the respective depth indicator 37 sothat, in use, the surgeon can determine how far the first directionaldilator 30 has been inserted into the patient. Similarly, as best shownin FIGS. 6A and 6B, the second directional dilator 40 preferablyincludes a plurality of second depth indicators 47 located on the outersurface 42 thereof. The plurality of second depth indicators 47 extend,on the outer surface 42 of the second directional dilator 40, generallyperpendicular to the second longitudinal axis 41. The plurality ofsecond depth indicators 47 indicate to the surgeon the various distancesbetween the second tip 40 a formed at the distal end 44 of the seconddirectional dilator 40 to the respective depth indicator 47 so that, inuse, the surgeon can determine how far the second directional dilator 40has been inserted into the patient. In the first preferred embodiment,the plurality of first and second depth indicators 37, 47 are spaced adistance of eighty millimeters (80 mm) to one hundred fifty millimeters(150 mm) from the first and second tips 30 a, 40 a in ten millimeter (10mm) increments. However, the plurality of plurality of first and seconddepth indicators 37, 47 are not limited to these dimensions and may bespaced from the first and second tips 30 a, 40 a at nearly any distanceor spacing that is preferred by a surgeon and is able to show the depththat the first and second directional dilators 30, 40 are inserted intothe patient.

In addition, the first and second directional dilators, 30, 40preferably include first and second grips 39, 49, respectively, locatedat the proximal ends 32, 42 thereof to better enable the surgeon to gripthe dilators 30, 40 in use. The first and second grips 39, 49 may beutilized by the surgeon during insertion, removal, twisting or otherwisemanipulating the first and second directional dilators 30, 40 duringsurgery.

As best shown in FIG. 1, the first directional dilator 30 preferably hasa first length Li while the second directional dilator 40 has a secondlength L2. The first length Li is preferably greater than the secondlength L2 to facilitate handling and insertion. Similarly, thestimulating probe 20 preferably has a probe length L3 such that theprobe length L3 is greater than the first length Li and the secondlength L2. The greater first length Li of the first directional dilator30 permits the proximal end 33 of the first directional dilator 30 toextend further out of the patient in the assembled and operationalconfigurations such that a surgeon may grasp the first grip 39 andremove or otherwise manipulate the first directional dilator 30 evenafter the second directional dilator 40 is inserted into the patient. Inthe first preferred embodiment, the first length is two hundred twentymillimeters (220 mm) and the second length is two hundred millimeters(200 mm), but are not so limited and may have nearly any length thatpermits insertion into the patient with the proximal ends 33, 43extending out of the patient. In addition, the first directional dilator30 preferably has a first diameter Di and the second directional dilator40 has a second diameter D2, the second diameter D2 is preferablygreater than the first diameter D1. In the first preferred embodiment,the first diameter Di is approximately seven and seven tenthsmillimeters (7.7 mm) and the second diameter D2 is approximatelyseventeen and one-half millimeters (17.5 mm). However, the first andsecond diameters Di, D2 are not so limited and may have nearly anydiameter desired by the surgeon for dilating tissue various distancesfrom the stimulating probe 20. Further, the first and second directionaldilators 30, 40 are not limited to having a circular cross-section andmay have nearly any cross-section and be adapted to shapes that permitsdirectional dilation in a manner that is preferred by a surgeon. Forexample, the first and second directional dilators 30, 40 may have anoval or oblong cross-sectional shape that urges dilation and a surgicalworking channel even further from a detected nerve 9 than a dilatorhaving a circular cross-section.

A method of using the stimulating probe 20 and first and seconddirectional dilators 30, 40 will now be described for accessing thepatient's spine 2. The technique may be particularly desirable foraccessing the lumbar region of the spine 2 via a lateral approach,although a similar or the same method may be used in other parts of thepatient's body.

Using the stimulating probe 20 and a triggered electromyograph (EMG) 50,the surgeon preferably maps a safe zone, i.e., a zone generally free ofany neural elements or nerves 9, on the tissue of interest (e.g., psoasmuscle 4). For example, on the psoas muscle 4, the anterior third of thepsoas muscle 4 is generally considered a safe zone.

Once a safe zone is established, anatomical placement is preferablyconfirmed via intra-operative fluoroscopy. The surgeon inserts thestimulating probe 20 through the psoas muscle 4 toward the patient'sspine 2. If the surgery is being performed on the intervertebral discspace 6, the distal end of the stimulating probe 20 may be inserted intothe annulus of the desired intervertebral disc space 6. Preferably, thestimulating probe 20 will be inserted via the most posterior portion ofthe safe zone.

The surgeon can insert or slide the first directional dilator 30 overthe stimulating probe 20 so that the first longitudinal axis 31 islocated to one side of the stimulating probe 20, preferably away from asensed neural element or nerve 9, through the psoas muscle 4 and into aposition proximate the patient's spine 2. The surgeon can then insertthe second directional dilator 40, if necessary, to further dilate thetissue proximate the outside surface 32 of the first directional dilator30 and further away from the sensed neural element or nerve 9. Thesurgeon can repeat this process as often as necessary. Finally, ifdesired, a retractor 60 can be inserted over the second directionaldilator 40 to subsequently retract the tissue and to permit removal ofthe first and second directional dilators 30, 40 and the stimulatingprobe 20. Alternatively, a working cannula (not shown) may be insertedover the second dilator 40 such that a procedure on the spine 2 may beperformed through the working cannula.

Additionally, if desired, before inserting the second directionaldilator 40, the stimulating probe 20 can be removed from the first bore35 and the dilator/probe combination rotated. Thereafter, using thetriggered EMG stimulation 50, the surgeon can verify that the nerve root9 is located at the expected side of the first directional dilator 30,preferably opposite the first channel 38. The stimulating probe 20 ispreferably re-inserted into the first bore 35, before insertion of thesecond directional dilator 40.

By using the first and second directional dilators 30, 40, as comparedto concentric sequential dilators as are generally known to those havingskill in the art, the directional sequential dilation system 10preferably ensures that the access opening is created away from theneural elements or nerves 9 of the psoas muscle 4, thus avoiding anyneural elements or nerves 9 that may, for example, be located on theposterior side of the stimulating probe 20. Moreover, the directionalsequential dilation system 10 also reduces the amount of tissue damagewhen separating the tissue by minimizing the amount of tissueseparation.

Alternatively, as shown in FIGS. 7-10, a one step blunt stimulatingdilator 100 comprising a second preferred embodiment of a dilationsystem of the present application may be used. The blunt stimulatingdilator 100 includes an outer surface 102, a proximal end 104, a distalend 106 and a bore 108 extending from the proximal end 104 to the distalend 106. The proximal end 104 includes an area 110 for attaching astimulating clip or cord. The distal end 106 includes an exposed,preferably pointed tip 112 for delivering electrical stimulation. Theouter surface 102 of the stimulating dilator 100 between the proximaland distal ends 104, 106 is preferably coated to prevent electricalleakage. The stimulating dilator 100 also preferably includes a channel114 formed in the outer surface 102 thereof for receiving a stimulatingprobe, such as the stimulating probe 20 illustrated in FIG. 1. Thestimulating probe can be any probe now or hereafter known fortransmitting an electrical pulse.

The stimulating dilator 100 offers the surgeon the ability tosimultaneously stimulate and dilate the psoas muscle 4. After placingthe tip 112 of the stimulating dilator 100 into the disc space, thestimulating probe 20 can be inserted through the channel 114 along theouter surface 102 of the dilator 100 to stimulate the periphery of thedilated tissue.

Alternatively, as shown in FIGS. 11 and 12, the stimulating dilator 100′comprised of a third preferred embodiment of the present application mayinclude a plurality of channels 114 formed in the outer surface 102thereof. For example, as shown, the stimulating dilator 100′ may includefour channels 114 a-d diametrically spaced on the outer surface 102 ofthe dilator 100′. In this manner, the surgeon can stimulate anterior,posterior, cranially, and caudally to verify the location of the nerveroot once the dilator 100′ is in place. Although as will be understoodby one of ordinary skill in the art, the stimulating dilator 100′ mayinclude any number of channels 114 including, for example, two, three,five or more.

A method of using the blunt stimulating dilation system will now bedescribed to produce access to the spine 2, in particular to provide anaccess opening through the psoas muscle 4 in the lumbar region of thespine 2 via a lateral approach. Although as will be understood by one ofordinary skill in the art, the method may be used in other parts of thebody and utilizing alternative approaches.

In use, a surgeon inserts, preferably laterally, the blunt stimulatingdilator 100, 100′ into the psoas 4 muscle via, for example, a twistingmotion. The surgeon preferably uses a triggered EMG 50 to transmit anelectrical pulse into the blunt stimulating dilator 100, 100′ in orderto locate a safe zone in the patient's psoas muscle 4 by locating nerveroots 9. Once the location of any nerve root 9 has been confirmed to beposterior to the blunt stimulating dilator 100, 100′, the surgeon canlaterally insert the blunt stimulating dilator 100, 100′ through thepsoas muscle 4 and toward the patient's spine 2, preferably into theannulus of the disc space 6. The surgeon inserts or slides thestimulating probe 20 into the channel 114 formed in the outer surface102 of the blunt stimulating dilator 100, 100′. If desired, the surgeonrotates the blunt stimulating dilator 100 with the stimulating probe 20located in the channel 114 while using the EMG 50 to verify the locationof the nerve root 9. Alternatively, in connection with the four channelblunt stimulating dilator 100′, rotation of the blunt stimulatingdilator 100 is not required. Rather, the stimulating probe 20 can beindependently inserted into each channel 114 a-d to verify the locationof the nerve root 9. The surgeon can then, if desired, insert aretractor over the stimulating dilator 100, 100′.

While the foregoing description and drawings represent the preferredembodiments of the present invention, it will be understood that variousadditions, modifications and substitutions may be made therein withoutdeparting from the spirit and scope of the present invention as definedin the accompanying claims. In particular, it will be clear to thoseskilled in the art that the present invention may be embodied in otherspecific forms, structures, arrangements, proportions, and with otherelements, materials, and components, without departing from the spiritor essential characteristics thereof. One skilled in the art willappreciate that the invention may be used with many modifications ofstructure, arrangement, proportions, materials, and components andotherwise, used in the practice of the invention, which are particularlyadapted to specific environments and operative requirements withoutdeparting from the principles of the present invention. In addition,features described herein may be used singularly or in combination withother features. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims, and notlimited to the foregoing description.

What is claimed is:
 1. A method of forming an access opening through apsoas muscle to a patient's spine, comprising: (a) laterally inserting astimulating probe into the psoas muscle, the stimulating probe having aprobe axis and a probe tip; (b) transmitting an electrical pulse intothe stimulating probe to locate a nerve and a safe zone in the psoasmuscle; (c) laterally inserting the stimulating probe through the psoasmuscle so the probe tip is proximate the patient's spine; (d) obtaininga first directional dilator having a longitudinal axis, a bore with alength extending from a proximal end to a distal end, a bore axis thatis offset from the longitudinal axis, and a channel formed in an outersurface thereof and extending from the proximal end to the distal end soas to be in communication with the bore along an entirety of the lengthof the bore; (e) inserting the first directional dilator through thepsoas muscle and toward the patient's spine guided by sliding engagementof the stimulating probe with the bore of the first directional dilatorand with the channel of the first directional dilator positioned betweenthe nerve and the longitudinal axis of the first directional dilator;(f) obtaining a second directional dilator having a longitudinal axis, abore with a length extending from a proximal end to a distal end, a boreaxis that is offset from the longitudinal axis, and a channel formed inan outer surface thereof and extending from the proximal end to thedistal end so as to be in communication with the bore along an entiretyof the length of the bore; and (g) inserting the second directionaldilator through the psoas muscle toward the patient's spine guided bysliding engagement of the first directional dilator with the bore of thesecond directional dilator and with the channel of the seconddirectional dilator positioned between the nerve and the longitudinalaxis of the second directional dilator.
 2. The method of claim 1,wherein in the step of inserting the first directional dilator, the boreaxis of the first directional dilator is coaxial with the probe axis. 3.The method of claim 1, wherein in the step of inserting the seconddirectional dilator, the bore axis of the second directional dilator iscoaxial with the longitudinal axis of the first directional dilator. 4.The method of claim 1, wherein the first directional dilator has a firstlength and the second directional dilator has a second length, andwherein the first length is greater than the second length so the firstdirectional dilator extends beyond the second directional dilator uponthe second directional dilator being inserted through the psoas muscle.5. The method of claim 4, wherein the stimulating probe has a probelength that is greater than the first length so the stimulating probeextends beyond the first directional dilator upon the first directionaldilator being inserted through the psoas muscle.
 6. The method of claim1, wherein the channel of the first directional dilator is radiallyaligned with the channel of the second directional dilator when thestimulating probe is received in the bore of the first directionaldilator and when the first directional dilator is received in the boreof the second directional dilator.
 7. The method of claim 1, furthercomprising the step of: inserting a retractor over the seconddirectional dilator.
 8. The method of claim 1, further comprising thestep of: confirming via intra-operative fluoroscopy the location of thesafe zone prior to step (c).
 9. A method of forming an access openingthrough a psoas muscle to a patient's spine, comprising: (a) laterallyinserting a stimulating probe into the psoas muscle, the stimulatingprobe having a probe axis and a probe tip; (b) transmitting anelectrical pulse into the stimulating probe to locate a nerve and a safezone in the psoas muscle; (c) laterally inserting the stimulating probethrough the psoas muscle so the probe tip is proximate the patient'sspine; (d) obtaining a first directional dilator having a firstlongitudinal axis, a first bore having a length extending from aproximal end to a distal end of the first directional dilator, a firstbore axis that is offset from the first longitudinal axis, and a firstchannel formed in an outer surface of the first directional dilator soas to be in communication with the first bore along at least a portionof the length of the first bore; (e) inserting the first directionaldilator through the psoas muscle and toward the patient's spine guidedby sliding engagement of the stimulating probe with the first bore ofthe first directional dilator and a portion of the stimulating probeextending into the first channel with the stimulating probe positionedbetween the nerve and the longitudinal axis of the first directionaldilator; (f) obtaining a second directional dilator having a secondlongitudinal axis, a second bore having a length extending from aproximal end to a distal end of the second directional dilator, a secondbore axis that is offset from the second longitudinal axis, and a secondchannel formed in an outer surface of the second directional dilator soas to be in communication with the second bore along at least a portionof the length of the second bore; and (g) inserting the seconddirectional dilator through the psoas muscle toward the patient's spineguided by sliding engagement of the first directional dilator with thesecond bore of the second directional dilator and a portion of the firstdirectional dilator extending into the second channel with the firstdirectional dilator positioned between the nerve and the secondlongitudinal axis of the second directional dilator.
 10. The method ofclaim 9, wherein in the step of inserting the first directional dilator,the first bore axis is coaxial with the probe axis.
 11. The method ofclaim 9, wherein in the step of inserting the second directionaldilator, the second bore axis is coaxial with the first longitudinalaxis.
 12. The method of claim 9, wherein the first directional dilatorhas a first length and the second directional dilator has a secondlength, and wherein the first length is greater than the second lengthso the first directional dilator extends beyond the second directionaldilator upon the second directional dilator being inserted through thepsoas muscle.
 13. The method of claim 12, wherein the stimulating probehas a probe length that is greater than the first length so thestimulating probe extends beyond the first directional dilator upon thefirst directional dilator being inserted through the psoas muscle. 14.The method of claim 9, wherein the first channel is radially alignedwith the second channel when the stimulating probe is received in thefirst bore and when the first directional dilator is received in thesecond bore.
 15. The method of claim 9, wherein the electrical pulse isgenerated by an EMG.
 16. The method of claim 9, further comprising thestep of: inserting a retractor over the second directional dilator. 17.The method of claim 9, further comprising the step of: confirming viaintra-operative fluoroscopy the location of the safe zone prior to step(c).